Since the seminal works of Fischer and Zingsheim (U. Ch. Fischer & H. P. Zingsheim, J. Vac. Sci. Technol., Vol. 19, pp. 881-885, 1981) as well as Deckman and Dunsmuir (H. W. Deckman & J. H. Dunsmuir, Appl. Phys. Lett., Vol. 41, pp. 377-379, 1982) surface-adsorbed colloidal particles have found wide-spread application as large scale masks for nanopatterning. As illustrated in FIG. 1, the masks can either be used in an etching process, where they protect the underlying surface (see FIG. 1(a)), or for material deposition, where they allow the formation of additional features only in the interstices between neighboring particles (see FIG. 1(b)). In case of the etching process, usually a thin film 1 is formed on a solid substrate 2 (step 1). After deposition of the colloidal mask 3, the film 1 is structured in a destructive treatment 4, such as reactive ion etching. Then, only those areas of the film 1 are stable, which are protected by the colloidal mask 3 (step 2). In case of the deposition process, the colloidal mask 3 is disposed on the substrate 2 (step 1). Then the thin film 1 is built up during the deposition 5 through the interstices in the mask 3 (step 2). Those two techniques yield complementary patterns. This basic concept of the etching process and the deposition process has been applied and modified in a number of different ways, as outlined in the following.
Initially, the colloidal particles were deposited on surfaces to form hexagonally dense-packed structures and nanostructures were formed by material deposition via thermal evaporation through the instices between the particles (Fischer & Zingsheim, Deckman & Dunsmuir, F. Burmeister et al., Langmuir, Vol. 13, pp. 2983, 1997). Alternative approaches applied first an etching step to shrink the particle size in hexagonally dense-packed particle layers. Then, material was deposited via evaporation onto the non-shadowed regions of the surface, thereby forming mesh-like nanostructures (C. Haginoya et al., Appl. Phys. Lett., Vol. 71, pp. 2934-2936, 1997; D.-G. Choi et al., Chem. Mater., Vol. 16, pp. 4208-4211, 2004). Later, the use of colloidal particles as a lithographic mask has been widened to less densely and less regularly arranged particles. For an overview over standard deposition schemes and the resulting colloidal masks, cf. to Himmelhaus & Takei and the references therein (M. Himmelhaus & H. Takei, Phys. Chem. Chem. Phys., Vol. 4, pp. 496 -506, 2002).
Boneberg et al. (J. Boneberg et al., Langmuir, Vol. 13, pp. 7080-7084, 1997) used the drying process of the colloidal suspension to form organic rings around the contact points of the spherical colloidal particles with the substrate by adding organic molecules to the suspension prior to drying. Aizpurua et al. (J. Aizpurua et al., Phys. Rev. Lett., Vol. 90, pp. 057401/1-4, 2003) used Argon ion beam etching to produce ring-like metal structures by first ablating the metal from the non-shadowed parts of the surface and then filling the shadowed area underneath the particles due to collisions with the Ar ions.
Van Duyne (C. L. Haynes et al., J. Phys. Chem. B, Vol. 106, pp. 1898-1902, 2002) utilized metal deposition onto colloidal masks at different deposition angles with respect to the surface to achieve differently formed nanostructures in the interstices of the colloidal masks.
Yang and coworkers (D.-G. Choi et al., J. Am. Chem. Soc., Vol. 127, pp. 1636-1637, 2005) used a dense-packed colloidal double layer for the fabrication of nanopores. A polystyrene (PS) latex suspension with an average particle diameter of 1 μm was mixed with a silica nanoparticle suspension with an average particle diameter of 50 nm. After deposition of the mixture onto a substrate and subsequent drying, a PS bead double layer embedded into a silica host matrix had formed. Subsequent steps of Reactive Ion Etching (RIE) led to bead removal and bead patterning as well as to the formation of silica nanopore structures.
Chilkoti and coworkers (W. Frey et al., Adv. Mater., Vol. 12, pp. 1515-1519, 2000) combined colloidal lithography on mica with a subsequent lift-off process to achieve ultraflat binary nanopatterns, lacking any topology despite of formation of the pattern. Such features are useful in all applications, where surface chemistry has to be properly distinguished from surface topology.
Ren and coworkers (Y. Wang et al., Nanotechnol., Vol. 16, pp. 819-822, 2005) used colloidal masks in combination with sputtering and evaporation deposition processes to form a complex secondary inorganic mask. The latter was then used for the preparation of triangular lattice arrays. This work is important because it discloses for the first time the difference between sputtering and evaporation onto convex-shaped particle layers.
In a recent work Himmelhaus and coworkers (J. Wright et al., Adv. Mater., Vol. 18, pp. 421-426, 2006) have utilized this difference between sputtering and evaporation in combination with the lift-off process as suggested by Chilkoti and coworkers to form ultraflat ternary patterns. The key to this technology is that sputtering coats the entire accessible substrate surface, leaving only those areas on the surface in direct contact with the colloidal particles uncoated, while evaporation coats only the non-shadowed regions. Accordingly, evaporation combined with subsequent sputtering forms a binary inorganic pattern on the substrate. Removal of the colloidal particles and backfilling of the residual apertures then yields the formation of a ternary structure, which becomes accessible via a subsequent lift-off process.
For deposition of inorganic materials, such as metals or metal oxides, mainly standard evaporation and sputtering processes have been applied as well known to those skilled in the art. However, Okazaki and Sambles (N. Okazaki & J. R. Sambles, A New Fabrication Technique and Current-Voltage Properties of a Au/LB/Au Structure, Extended Abstracts, Intl. Symposium on Organic Molecular Electronics, Nagoya, Japan, 18-19 May 2000, pp. 66-67) and later Peterson and coworkers (R. M. Metzger et al., J. Phys. Chem. B, Vol. 105, pp. 7280-7290, 2001) used metal evaporation at a base pressure much higher than usual (up to about 5×10−3 hPa) to achieve soft landing of the evaporated metal atoms onto an ultrathin organic film. This was achieved by elevating the low base pressure of the evaporator in use (˜10−6 hPa) by means of argon. Prior to this, the substrate bearing the ultrathin organic film on one surface had been mounted inside of the evaporation chamber with the coated surface facing away from the evaporation source. Accordingly, only those metal atoms that were backscattered due to collisions with the argon atoms could be deposited on the organic film. Due to the low impact of the backscattered atoms, the organic film was not damaged during the deposition process. These activities aim at the fabrication of metal-organic film-metal sandwich layers for applications in molecular electronics.